The assignee common to this application previously proposed a bulky sheet including a fiber aggregate formed by hydroentangling a fiber web and having a plurality of projections and depressions (see patent literature 1 below). The bulky sheet is produced by subjecting a fiber aggregate obtained by hydroentangling a fiber web to a second hydroentanglement treatment on a patterning member having a plurality of projections and depressions and a plurality of perforations. The bulky sheet obtained by this method has a plurality of projections that provide flexibility and good hand and is therefore suited for use as a cleaning sheet. Moreover, the method produces a bulky sheet at a low cost. When used as a cleaning sheet, the bulky sheet is capable of trapping and holding fine dust adhering to the surface being cleaned between constituent fibers.
Apart from the above technique, patent literature 2 below discloses a nonwoven fabric wiper composed of a fibrous material and having a plurality of undulations on at least one side thereof. According to the literature, the wiper is produced through the following steps (a) to (c):    (a) forming a stack composed of at least one hydrophilic fiber web and at least one thermally self-crimping hydrophobic fiber web,    (b) directing high pressure jets of water from fine orifices to the stack placed on a support screen having a continuous flat portion and a plurality of discretely distributed projections and/or recesses and a plurality of fine drain apertures thereby to entangle and re-arrange the fibers of the two kinds of webs to provide a nonwoven fabric having fiber density unevenness in its planar direction, and    (c) dewatering and/or drying the nonwoven fabric, followed by heat treatment to crimp the synthetic fibers.
A wiper having a plurality of undulations with a relatively large surface level difference is obtained by the method of patent literature 2. However, the need to use two kinds of fibers—hydrophilic fibers and thermally self-crimping hydrophobic fibers, and to conduct heat treatment to cause the thermally self-crimping hydrophobic fibers to self-crimp makes the processing steps complicated, which is economically disadvantageous.
Patent literature 3 discloses a nonwoven fabric having at least one of predetermined groove portions, openings, and protrusions that is obtained by directing a fluid mainly comprising gas onto a side of a fiber aggregate having a sheet form placed on a prescribed air-permeating support. The fiber aggregate contains thermoplastic fibers that soften at a prescribed temperature. The production of this nonwoven fabric involves softening the thermoplastic fibers by heating. This makes the processing steps complicated and is economically disadvantageous.
Patent literature 4 discloses a non-apertured cleaning sheet the working face of which comprises nonrandom raised regions and recessed regions. The recessed regions form a continuous pattern in the X-Y dimension surrounding discrete raised regions, and the continuous pattern consists of channels. The working face has an average height differential of at least about 1 mm and a total pore volume of greater than 750 gsm. The method for making the cleaning sheet involves the step of hydroentangling the fibers of a nonwoven structure on a forming belt having a desired pattern of raised and recessed regions. This makes the processing steps complicated and is economically disadvantageous. Furthermore, the recessed regions of the cleaning sheet have disadvantageously low capability of trapping large particles.